Alloy steel powder for metal injection molding improved in sintering characteristics and sintered article

ABSTRACT

An alloyed steel powder for metal injection molding that eliminates the problems of decreased product strength and difficulty of temperature control which exist in conventional alloys for sintering and that improves productivity of the sintering furnace is provided, together with a sintered body thereof. This is an alloyed steel powder for metal injection molding which consists as mass percentages of 0.1 to 1.8% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11 to 18% Cr, 2 to 5% Nb and the remainder Fe and unavoidable impurities, and which may further comprise 5.0% or less of at least one of Mo, V and W, or a sintered body (wherein C is 0.1 to 1.7%) of these powders.  
     As shown in FIGS.  6  through  9 , the alloyed steel powder for metal injection molding of the present invention results in a sintered body with a constant sintered density over a 50° C. range of sintering temperatures, thereby facilitating sintering temperature control and improving productivity.

TECHNICAL FIELD

The present invention relates to an alloyed steel powder for metalinjection molding (MIM) which is effective to realize comp ex-shapedparts of very hard, highly corrosion resistant martensite stainlesssteel or tools of alloyed steel with good dimensional precision, andrelates to a sintered body.

BACKGROUND ART

SKD11, SUS420, SUS440C and the like have conventionally been used asmetal injection molding powders for obtaining very hard, highlycorrosion resistant sintered bodies. These steels in which hardness isobtained by mainly use of Cr carbide exhibit an austenite phase in thesintering temperature range, and have a poor degree of sintering becausethe speed of elemental movement (diffusion) which promotes sintering isslower than in a ferrite phase. On the other hand, if the temperature israised to the temperature at which a liquid phase appears in order topromote sintering, a large amount of liquid phase arises at once,carbides are formed as networks at the grain boundaries, and either thestrength of the product is seriously diminished or it is deformed to thepoint that the shape of the product cannot be maintained. To avoidthese, it is necessary to proceed with the sintering temperaturecontrolled within an extremely narrow temperature range of ±5° C. or inother words about 10° C. Because of this, it has been necessary to limitthe usable region of the sintering furnace, sacrificing productivity.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to eliminate the aforementioneddiminishment of product strength and difficulty of controlling sinteringtemperature which are problems of the aforementioned conventionalsintering alloys, and to provide an alloyed steel powder for metalinjection molding and a sintered body which contribute to enhancingproduct characteristics and improving productivity of the sinteringfurnace.

In order to solve the aforementioned problems, the present invention hasthe following constitution.

(1) An alloyed steel powder for metal injection molding with improveddegree of sintering, consisting as mass percentages of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0% Nb, and aremainder Fe and unavoidable impurities.

(2) An alloyed steel powder for metal injection molding with improveddegree of sintering, consisting as mass percentages of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 5.0% or less of atleast one of Mo, V and W, 2.0 to 5.0% Nb, and a remainder Fe andunavoidable impurities.

(3) An alloyed steel powder for metal injection molding with improveddegree of sintering according to (2) above, wherein the at least one ofMo, V and W is 0.3 to 1.6%.

(4) An alloyed steel sintered body for metal injection molding withimproved degree of sintering, consisting as mass percentages of 0.1 to1.7% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0%Nb, and a remainder Fe and unavoidable impurities.

(5) An alloyed steel sintered body for metal injection molding withimproved degree of sintering, consisting as mass percentages of 0.1 to1.7% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 5.0% or lessof at least one of Mo, V and W, 2.0 to 5.0% Nb, and a remainder Fe andunavoidable impurities.

(6) An alloyed steel sintered body for metal injection molding withimproved degree of sintering according to (5) above, wherein the atleast one of Mo, V and W is 0.3 to 1.6%.

The focus of the present invention is on producing a Nb carbide with lowdiffusion by adding Nb to a steel alloyed primarily with Cr carbide.Because this Nb carbide has a low diffusion speed it is unlikely to bulkby diffusion during sintering of the metal injection molded product, andthe Cr carbide is also precipitated around the core of this Nb carbide.

Using the pinning effect of this Nb carbide it is possible to preventfrom bulking and network formation of the carbide more effectively thanwhen only the Cr carbide is present.

In the constitution of the present invention, C forms carbides andcontributes hardness, resulting in a martensite structure. 0.1 to 1.8%is desirable as the amount range of C in the powder. The sinteringtemperature and sintered density vary according to the amount of C.Consequently, graphite is added appropriately during molding of thepowder to adjust the amount of C in the sintered product to 0.1 to 1.7%.A sintered body with a high sintered density can be manufactured undereasy temperature control. The lower limit of 0.1% in both powder andsintered body was set because that is the minimum amount necessary toproduce the aforementioned Nb carbide and because it is the minimumamount at which the C would dissolve in the matrix to form martensite.The upper limits of 1.8% in the powder and 1.7% in the sintered bodywere set considering the amount of C that is lost from the powder duringsintering because at this level C contributes to hardness by forming aCr carbide in the sintered body, but above 1.7% hardness is not furtherimproved but on the contrary toughness (transverse rupture strength) isdiminished.

Si improves deoxidation and hot water flow. If the amount is less than0.3%, the oxygen amount rises and hot water flow is adversely affected,while if it is more than 1.2%, hardenability is adversely affected.

If Mn is less than 0.1%, hot water flow is adversely affected, while ifit is over 0.5%, it combines with oxygen, increasing the amount ofoxygen in the powder. Consequently, it is set in the range of 0.1 to0.5%.

Cr improves hardenability and increases hardness by producing carbides.It also dissolves in the matrix including the carbides, thereby, itimproves corrosion resistance. A range of 11.0 to 18.0% is desirable.

Mo, V and W produce carbides, and together with Nb have a pinning effecton the Cr carbides therefore they enhance the strength and hardness ofthe sintered body. If these are more than 5.0%, toughness will bediminished so 5.0% or less is desirable, and a range of 0.3 to 1.6% ismore desirable from the standpoint of hardenability andcost-effectiveness. A noticeable improvement in hardness is difficult toachieve below 0.3%, while more than 1.6% is not cost-effective.

Nb controls diffusion of Cr carbides and improves hardenability by meansof the pinning effect of low-diffusion Nb carbides. By adding 2.0 to5.0% Nb, it is possible to improve the productivity of the sinteringfurnace because the sintering temperature needs only to be controlledwithin ±25° C. rather than within ±5° C. as it does conventionally. Thiseffect isn't sufficient if the amount of Nb is less than 2.0%, while ifit exceeds 5.0%, the amount of oxygen increases and moldability isadversely affected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a pattern of sintering performed in an example of thepresent invention.

FIG. 2 is a graph of the sintering characteristics of SKD11.

FIG. 3 is a graph of the sintering characteristics of SUS420.

FIG. 4 is a graph of the sintering characteristics of SUS440C.

FIG. 5 is a graph of the sintering characteristics of ComparativeExample 1.

FIG. 6 is a graph of the sintering characteristics of Example 1 of thepresent invention.

FIG. 7 is a graph of the sintering characteristics of Example 2 of thepresent invention.

FIG. 8 is a graph of the sintering characteristics of Example 3 of thepresent invention.

FIG. 9 is a graph of the sintering characteristics of Example 4 of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The samples shown in Table 1 below were prepared and their sinteringcharacteristics tested. TABLE 1 Steel Composition (%) Dm T/D type C SiMn Cr Mo V W Nb O Fe (μm) (g/cm³) SKD11 1.66 0.34 0.44 11.80 1.02 — — —3300 Remainder 11.90 4.04 SUS420 0.27 0.85 0.33 13.09 0.59 — — — 3200Remainder 10.01 4.30 SUS440C 0.96 0.91 0.18 17.12 0.05 0.07 — — 2700Remainder 9.72 4.21 Comp. 0.60 0.73 0.47 12.53 1.49 — — 0.34 3900Remainder 10.22 4.27 Example 1 Example 1 1.03 0.92 0.22 17.01 — — — 3.014100 Remainder 9.92 4.17 Example 2 0.66 0.88 0.44 12.18 1.42 — — 3.224200 Remainder 10.98 4.18 Example 3 0.96 0.87 0.21 17.12 0.41 0.17 0.082.99 3400 Remainder 9.86 4.08 Example 4 0.56 0.93 0.31 12.34 0.50 — —2.81 2500 Remainder 9.92 4.17 Comp. 0.65 0.89 0.45 12.15 1.46 — — 7.3313500  Remainder 10.34 4.20 Example 2

The C amount of each sample was adjusted. Graphite powder was added withthe aim of achieving C amounts after sintering of 1.30%, 1.50% and 1.70%for SKD11, 0.30%, 0.50%, 0.70% and 0.90% for SUS420, 1.30% for Example1, 0.75%, 0.80%, 1.00% and 1.20% for SUS440C, 0.50%, 0.70% and 0.90% forComparative Example 1 and Example 2, 1.30% for Example 3 and 0.90% forExample 4. A sintering test was not performed in the case of ComparativeExample 2 because the amount of oxygen was too great at the powderstage.

The sintering test was performed as follows.

A suitable amount of graphite powder was added to each of the metalpowders shown in Table 1, based on the target amount of C aftersintering, 5.0 wt % of stearic acid (outer number) was added to thepowder, and uniform kneading was performed with heating at 80° C.

The kneaded products were cooled to room temperature, and the solidifiedpellets were pulverized. The pulverized pellets were press molded at amolding pressure of 0.6 Ton/cm² (ø11.3×10t, N=3).

Sintering was performed according to the pattern shown in FIG. 1. InFIG. 1, the sintering temperatures were the appropriate temperaturesshown in Tables 2 through 5, such as 1370° C., 1390° C. and 1410° C.

Tables 2 through 5 show the sintered density of each sample at differentsintering temperatures and for different target amounts of carbon aftersintering. The amounts of C, O and N in the sintered products of eachsample are shown at the bottom of Tables 2 through 5, along withsintered hardness (Hv) in the case of Tables 4 and 5. The sinteringcharacteristics shown in Tables 2 through 5 are also shown in graph formin FIGS. 2 through 9.

The structures were observed and the hardness of the sintered bodies wasmeasured to determine the respective appropriate control ranges ofsintering temperature. The appropriate control range of sinteringtemperature was the sintering temperature range within which thesintered density remained nearly constant within a range of ±0.1 g/cm³as the sintering temperature rose on the sintering temperature-sintereddensity graph. TABLE 2 SKD11 SUS420 Target C amount (%) Target C amount(%) after sintering after sintering Steel type 1.30 1.50 1.70 Steel type0.30 0.50 0.70 0.90 Molded product 4.91 4.90 4.88 Molded product 4.854.81 4.78 4.76 Density density Sintering 1220 — — 6.84 Sintering 1250 —— 6.75 7.07 Tempera- 1230 — 6.71 7.25 Tempera- 1270 — — 6.82 7.47 ture(° C.) 1240 6.81 7.20 7.61 ture (° C.) 1290 — — 7.06 7.78 1250 7.21 7.587.69 1310 6.82 — 7.38 7.91 1260 7.68 7.70 7.69 1330 6.84 6.98 7.79 —1270 7.71 7.69 — 1350 6.86 7.27 7.85 — — — — — 1370 6.92 7.70 — — — — —— 1390 7.41 7.69 — — — — — — 1410 7.70 — — — C (%) 1.28 1.47 1.66 C (%)0.33 0.57 0.79 0.99 O (ppm) 11 10 11 O (ppm) 17 40 27 41 N (ppm) 7 8 9 N(ppm) 3 4 1 3

TABLE 3 SUS440C Comparative Example 1 Target C amount (%) Target Camount (%) after sintering after sintering Steel type 0.75 0.80 1.001.20 Steel type 0.50 0.70 0.90 Molded 5.01 5.00 4.96 4.94 Molded product4.68 4.69 4.69 product density density Sintering 1230 — — 6.72 6.70Sintering 1270 5.44 6.23 7.38 tempera- 1240 6.88 6.91 6.88 6.93 tempera-1290 5.71 6.92 7.77 ture(° C.) 1250 6.93 6.94 7.00 7.10 ture(° C.) 13106.50 7.75 7.77 1260 6.97 7.00 7.19 7.52 1330 7.31 7.76 — 1270 7.03 7.127.61 7.63 1350 7.77 — — 1280 7.14 7.26 7.64 — 1370 7.77 — — 1290 7.247.41 7.63 — — — — — 1300 7.36 7.56 — — — — — — — — — — — — — — — C (%)0.84 0.86 1.04 1.24 C (%) 0.54 0.76 0.96 O (ppm) 130 60 42 34 O (ppm) 2114 20 N (ppm) 7 7 5 6 N (ppm) 3 2 13

TABLE 4 Example 1 Example 2 Target C amount (%) Target C amount (%)after sintering after sintering Steel type 1.30 Steel type 0.50 0.700.90 Molded product density 4.41 Molded product density 4.56 4.55 4.56Sintering 1240 6.34 Sintering 1290 5.88 6.12 6.44 tempera- 1250 7.10tempera- 1310 6.79 6.98 7.27 ture (° C.) 1260 7.68 ture (° C.) 1330 7.767.76 7.76 1270 7.69 1350 7.76 7.75 7.75 1280 7.70 1370 7.77 7.76 7.771290 7.70 — — — — 1300 7.69 — — — — 1310 7.70 — — — — — — — — — — C (%)1.25 C (%) 0.52 0.73 0.94 O (ppm) 11 O (ppm) 26 22 32 N (ppm) 7 N (ppm)10 8 7 Sintered hardness (Hv) 700 Sintered hardness (Hv) 600 640 310

TABLE 5 Example 3 Example 4 Target C amount (%) Target C amount (%)after sintering after sintering Steel type  1.30 Steel type  0.90 Moldedproduct 4.85 Molded product 4.85 density density Sintering 1230 —Sintering 1300 6.84 temperature 1240 6.37 temperature 1310 7.25 (° C.)1250 7.14 (° C.) 1320 7.58 1260 7.71 1330 7.83 1270 7.72 1340 7.83 12807.72 1350 7.83 1290 7.72 1360 7.79 1300 7.71 1370 7.77 1310 7.72 13807.75 C (%) 1.35 C (%) 0.94 O (ppm) 46 O (ppm) 11 N (ppm) 28 N (ppm) 9Sintered hardness 749 Sintered hardness 680 (Hv) (Hv)

As discussed above, in the alloyed steel powder for metal injectionmolding of the present invention containing Nb, the appropriatesintering temperature control range is greater than in the case ofSKD11, SUS420 and SUS440C. That is, while the appropriate sinteringtemperature control range is about 10° C. in the case of SKD11, SUS420and SUS440C, in the present invention it is broadened to about 50° C.,facilitating sintering temperature control and improving productivity.The sensitivity to C value after sintering is also weaker, and almostthe same sintering characteristics (temperature vs. density) areobtained with C values of 0.5 to 0.9%.

1. An alloyed steel powder for metal injection molding with improveddegree of sintering, consisting as mass percentages of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0% Nb, and aremainder Fe and unavoidable impurities.
 2. An alloyed steel powder formetal injection molding with improved degree of sintering, consisting asmass percentages of 0.1 to 1.8% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0to 18.0% Cr, 5.0% or less of at least one of Mo, V and W, 2.0 to 5.0%Nb, and a remainder Fe and unavoidable impurities.
 3. An alloyed steelpowder for metal injection molding with improved degree of sinteringaccording to claim 2, wherein the at least one of Mo, V and W is 0.3 to1.6%.
 4. An alloyed steel sintered body for metal injection molding withimproved degree of sintering, consisting as mass percentages of 0.1 to1.7% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0%Nb, and a remainder Fe and unavoidable impurities.
 5. An alloyed steelsintered body for metal injection molding with improved degree ofsintering, consisting as mass percentages of 0.1 to 1.7% C, 0.3 to 1.2%Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 5.0% or less of at least one ofMo, V and W, 2.0 to 5.0% Nb, and a remainder Fe and unavoidableimpurities.
 6. An alloyed steel sintered body for metal injectionmolding with improved degree of sintering according to claim 5, whereinthe at least one of Mo, V and W is 0.3 to 1.6%.